I cannot vouch for the quality of one brand vs the other, but Li-Ion is just an abbreviation for Lithium-Ion. I think there is a battery thread that will talk about what type of cells, quality etc of one brand or another.

Yep, 5600 mah is 400 mah more than 5200 mah. Milliamp hours are a measure of the total electric charge the battery can hold. The amount of current the light will draw is based on the internal resistance (brightness) so for a given brightness more mah means it will last longer. Always get the highest mah/$.